A human spine comprises 33 vertically aligned bone structures termed “vertebrae” or “vertebral bodies”: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused into one bone, the sacrum), and 4 coccygeal (fused into one bone, the coccyx). Each vertebral body comprises a cylindrical structure composed of hard cortical bone on the outside and less dense cancellous bone on the inside, while the inferior and superior sides of each vertebral body are layered with end plates that are typically about 1 mm thick and comprise both bone and hyaline cartilage. Between each vertebral body is a flexible, connective tissue termed an “intervertebral disc” which secures one vertebral body to another and functions as a shock absorber. Degenerative discs are normally associated with the gradual breakdown of the structure of the disc and spinal canal due to age. Disk herniation is a rupture, often due to an acute injury, of fibrocartilagenous material (annulus fibrosis) that surrounds the intervertebral disk, and the release of the disk's center portion containing a gelatinous substance called the nucleus pulposus. Pressure from the vertebrae superior and inferior to the disc may cause the nucleus pulposus to be forced outward, placing pressure on a spinal nerve and causing considerable pain and damage to the nerve.
Spinal fixation is a surgical technique in which the damaged disc is removed between two vertebral bodies and replaced by a spinal implant to facilitate spine fusion between the bodies, while also stabilizing and strengthening the spine, and to mimic the cushioning effects of an endogenous disc. Disorders treated using spinal implants include degenerative disc disease, scoliosis, kyphosis, spondylolisthesis, and fracture.
Types of spine fusion or fixation surgery comprise: anterior cervical discectomy and fusion (ACDF) for the neck; and posterior and anterior lumbar spine fusion for the thoracic, lumbar, and sacral spine, wherein the terms “anterior” and “posterior” refer to the point of entry into the spine by the surgeon. Spine fusion surgeries can also be classified by “levels”, meaning the number of intervertebral discs that are removed and fused together. For example, “one level” surgery refers to the removal of one disc and the fusion of two adjoining vertebral bodies, while a “two level” surgery refers to the removal of two consecutive discs and the fusion of the three vertebral bodies adjoining the discs.
Fusion implants generally comprise rods, plates, screws, interbody cages, and intervertebral spacers. The implants may be used with or without a bone autograft or allograft inserted in the disc space. Interbody cages are small hollow devices with perforated walls in which a bone autograft or allograft, or bone morphogenic protein (BMP) is inserted into the cage to promote bone fusion between the endplates of the inferior and superior vertebral bodies. Unfortunately, cages have been shown to have result in a high degree of subsidence, wherein subsidence of the cage was defined as a decrease in total vertical height of the two fused vertebral bodies as measured on the lateral cervical radiographs made 3 and 6 months postoperatively compared with the directly postoperative radiographs. (van Jonbergen HP, et al. “Anterior cervical interbody fusion with a titanium box cage: early radiological assessment of fusion and subsidence” Spine J. 2005 November-December; 5(6):645-9; discussion 649). In another study, the use of cage and plate construct in 2-level ACDF (Group B) resulted in a shorter fusion duration and a lower subsidence rate than that of cage alone (Group A), and wherein use of cage alone resulted in a subsidence rate of 35.71% (10/28) of group A as compared with 11.54% (3/26) of group B). (Oh J K, et al. “Stand-alone Cervical Cages Versus Anterior Cervical Plate in 2-Level Cervical Anterior Interbody Fusion Patients: Clinical Outcomes and Radiologic Changes”, J Spinal Disord Tech. 2012 Feb. 23). Additionally, BMP has been labeled dangerous for use in ACDF by the US Food and Drug Administration because it may trigger swelling of neck tissues.
Spacers, which are biocompatible devices, are inserted into the disc space to promote spine fusion and stability, as well as to keep the spine from compressing the spinal nerves. The spacer is held in place by the pressure of the superior and inferior vertebral bodies, and can easily migrate posteriorly into the spinal canal and cause compression and pain from impingement of the spinal cord or nerves. This approach also has disadvantages in that attachment to the vertebral endplates is not as strong as attachment to the hard anterior cortex of the vertebrae, as seen in flat plates.
Flat plates, such as an anterior cervical plate for use in ACDF, are affixed to the anterior cortex of the spine by screws placed into superior and inferior vertebral bodies (see FIG. 1), and the plate may be used with or without a intervertebral spacer, bone graft, and/or cage. When used with a graft, the anterior cervical plate is generally effective in preventing interbody graft dislodgement toward the esophagus (which causes ostoperative dysphagia), and it also enhances spinal fusion by providing fixation between the vertebral bodies. But, this method can be disadvantageous as these plates are generally not low-profile, resulting in post-operative dysphagia, and recurrent laryngeal nerve palsy (Fountas K N, et al. Anterior cervical discectomy and fusion associated complications. Spine (Phila Pa 1976). 2007 Oct. 1; 32(21):2310-7). ACDF plates also tend to be long and may encroach upon the space of an adjacent disc space or vertebral body, which may lead to disc degeneration. And when an ACDF plate is used with a spacer, the surgeon first implants the spacer into the intervertebral space, and then tries to mix and match a plate that will fit the size of the spacer. This is a tedious and cumbersome procedure to perform during surgery.
Accordingly, there is a need in the art for improved spinal plates and spacer assemblies that have one or more of the following characteristics: a lower profile plate to reduce the risk for post-operative dysphagia; that promotes implant stability by preventing slippage and/or rotation of the bone graft and/or spacer within the interbody space; that prevents the migration of a interbody spacer and graft into the spinal canal; and/or that provides a tight fit between the spacer and graft and the adjoining superior and inferior endplates so as to promote spine fusion.